3rd Generation Partnership Project (3GPP) long term evolution (LTE) system, introduced as 3GPP release 8, is an improved universal mobile telecommunication system (UMTS). An LTE system offers high peak data rates, low latency, improved system capacity, and low operating cost resulting from simple network architecture. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) communicating with a plurality of mobile stations, referred as user equipment (UE).
3GPP introduces Minimization of Drive Test (MDT) to give service provider a cost effective way for coverage optimization. Radio layer coverage in a mobile system may vary depending on a location of a base station, deployment of buildings nearby, interferences during the usage of a mobile user and other environmental factors. Traditionally, service providers need to perform drive test to collect measurement result and location information. The collected information is then used to analyze the coverage. Optimizations are done by adjusting parameters based on the analysis. After each optimization, drive test has to be undertaken again to make sure the impact of the changes on the system is positive. Such tests are very costly. MDT is designed to alleviate the problem by providing a method to use UEs to perform such test. It is beneficial to collect UE measurement results on the field to enable a more efficient network optimization and it is feasible to acquire measurement results, location information and other relevant information from the UEs.
There are two different types of MDT: immediate MDT and logged MDT. In immediate MDT cases, a UE is configured to perform measurement in Radio Resource Control (RRC) Connected state. The measurement report is sent to an eNB immediately at the time of the reporting condition. In logged MDT cases, a UE is configured to perform measurement in RRC Idle state when some configured conditions are satisfied. The UE stores measurement logs and reports to an eNB at a later point of time. In either type of the MDT, how to accurately interpret the measurements and correlate different information are important to get an accurate coverage picture.
In 3GPP Release-10, there are two measurements included in MDT for the purpose of uplink (UL) coverage and performance characterization: Power Headroom (PHR) measurement by UE, and uplink signal strength and signal-to-interference plus noise ratio (SINR) measurement by eNB. PHR can be used by the eNB to calculate the path loss of the UE, which is then used in setting for SINR target. PHR indicates how much power UE is left with to start using full power, which is the difference between the current UE transmit power and the maximum UE transmit power. There are some issues with the current art in using the UL measurements for MDT.
First, uplink measurement itself is not sufficient to build an UL performance map. Current art suggests that a low PHR value is an indicator of a UL performance problem. Such predication is not accurate and is wrong in some cases. The used power level for transmission is dynamically dependent on Modulation and Coding Scheme (MCS) and the bandwidth—the number of Physical Resource Blocks (PRBs). Therefore, for services with dynamic bit rate, a low PHR is not necessarily a sign of uplink problem. For example, if a service is using a high bit rate, the PHR may be low because the base station is aggressive in the link adaptation. A low PHR may also be a result of a base station prioritizing a certain UE by allocating a wide bandwidth to this UE. Current art also suggests that a low UL signal strength or SINR is an indicator of uplink performance problems. Such predication may also be wrong. Under current link-adaptation and power control method, receive power measurement together with interference measurement in certain circumstances can represent UL performance. However, these measurements do not tell everything about coverage problems. One type of coverage problem occurs when a UE is power limited and cannot achieve a planned minimum bit rate. Many factors can result in a low UL signal strength, such as scheduled low bit rate. Low UL signal strength can also be required in order to reduce UL interference, or to save UE battery. Base stations could also choose to schedule UEs with conservative link adaptation and low power. Therefore, low signal strength is not sufficient to indicate an uplink coverage problem.
A second issue with the UL measurement for MDT is how to correlate DL measurements, location information and UL measurements. Current art suggests that location information is useful and that DL measurements may be useful, however, there has been no analysis of how to provide and correlate these information. Collecting DL measurements, location information and UL measurements are matured art if performed separately. However, there exists discrepancy in collecting the information. DL measurements can be collected all the time since the reference transmissions are always ongoing. UL measurements, on the other hand, can only be collected when data transmission occurs. Therefore, for MDT purposes, many DL measurements and location information have no relevance for UL observations.
The present invention addresses two main issues in the current art for MDT. The first issue is how to correlate UL measurement data with QoS information to find UE performance problems. The second issue is how to better schedule and correlate DL measurement, location information, and UL measurement to improve MDT measurement and logging efficiency.